Obtaining useful chemicals, fuels, and energy from renewable biomass represents an important challenge as conventional fossil sources of these materials are slowly depleted. Lignocellulosic biomass is being studied widely as a viable feedstock for renewable liquid biofuels and chemicals because of its low cost and global availability. Biomass-derived fuels and chemicals are projected to substantially reduce net CO2 emissions as well, if produced with minimal use of fossil fuels.
To meet this challenge, there have been extensive efforts to convert biomass to fuels and other useful chemicals. Producing fuels and chemicals from biomass requires specialized conversion processes different from conventional petroleum-based conversion processes due to the nature of the feedstock and products. High temperatures, solid feed, high concentrations of water, unusual separations, contaminants, and oxygenated by-products are some of the features of biomass conversion that are distinct from those encountered in petroleum upgrading. Thus, there are many challenges that must be overcome to efficiently produce chemicals from biomass.
Lignocellulosic biomass (wood, grasses, agricultural residues, etc.) is an alternative, renewable, and sustainable source of feed with significant potential to address the increasing demands for alternative liquid fuels and ‘green’ chemicals. These feedstocks do not directly compete with the food supply, but have limited utility due to their inherent characteristics and storage limitations. Feedstock supply and the logistics of lignocellulosic biomass upgrading are challenging due to the low bulk density, low energy density, and high ash content of the feed. The chemical and physical inconsistencies of feedstocks are substantial barriers that limit the ability of designing a single, widely applicable process for the upgrading of biomass to fuels and chemicals.
Biomass materials generally comprise cellulose (35%-60%), hemicellulose (15%-40%) and lignin (10%-40%) as major components, a variety of lesser organic materials, water, and some mineral or metallic elements. A range of biomass derived materials can be pyrolyzed to produce mixtures of hydrocarbons, oxygenates, CO, CO2, water, char, coke, and other products. A particularly desirable form of pyrolysis is known as catalytic fast pyrolysis (CFP) that involves the conversion of biomass in a fluid bed reactor in the presence of a catalyst. The catalyst is usually an acidic, microporous crystalline material, usually a zeolite. The zeolite is active for the upgrading of the primary pyrolysis products of biomass decomposition, and converts them to aromatics, olefins, CO, CO2, char, coke, water, and other useful materials. The aromatics include benzene, toluene, xylenes, (collectively BTX), and naphthalene, among other aromatics. The olefins include ethylene, propylene, and lesser amounts of higher molecular weight olefins. BTX aromatics are desirable products due to their high value and ease of transport.
The minerals or metallic elements present as contaminants in biomass, sometimes collectively referred to as alkali and alkaline earth elements (AAEMs) although they may contain many other elements, present a challenge to catalytic processes. These elements can deactivate the catalyst or interfere with the smooth operation of a CFP process by a number of mechanisms. It is thus desirable to limit the amount of the AAEMs that are introduced into the CFP process, or remove the AAEMs, or both, in order to provide a commercially viable process for upgrading biomass to fuels and chemicals. Other impurity elements, primarily sulfur and nitrogen, present in biomass are also detrimental to the conversion of biomass to useful chemicals and fuels. Sulfur and nitrogen can inhibit catalyst activity, complicate product purification, and contaminate effluent streams. Processes for removing sulfur and nitrogen are also needed. The present invention addresses methods to reduce impurities including the AAEMs and sulfur and nitrogen in biomass feed to a CFP process.
In U.S. Pat. No. 8,022,260, a process is described that utilizes an activating step of introducing an additive to make a biomass more susceptible to conversion, and then converting the activated biomass to a product comprising bio-oil. Magnesium and aluminum salts are introduced into the biomass in a wet milling step in one example.
U.S. Patent Application Publication 2013/0340746 describes a process for converting AAEMs present in biomass into thermally stable, catalytically inert salts using hydrochloric, sulfuric, or phosphoric acids in preparation for a biomass pyrolysis process.
In U.S. Pat. No. 8,168,840, a process is described comprising: (i) swelling biomass with a solvent, optionally aided by pH control, application of mechanical action, the incorporation of additive(s), and temperature control; (ii) removing solvent from the swollen solid biomass material by applying mechanical action to the solid biomass material to form a solid modified lignocellulosic biomass material having an increased bulk porosity; and (iii) subjecting the solid modified lignocellulosic biomass material to enzymatic hydrolysis, thermoconversion, or combinations thereof. Optionally the material can be modified by incorporation of a soluble catalyst before it is upgraded. Catalytically upgrading of the swollen, modified, and dried biomass in a fixed or fluid bed of solid catalyst is not discussed.
In U. S. Patent Application Publication 2012/0301928, a method is described for pretreating lignocellulosic biomass prior to hydrolysis, comprising: immersing lignocellulosic biomass in water to swell the biomass; wet-milling the swelled biomass; and popping the wet-milled biomass. Neither minerals removal nor catalytic pyrolysis is mentioned. In U.S. Patent Application Publication 2014/0161689, a process is described for digesting biomass to remove sulfur or nitrogen compounds, reforming the resulting solution with a soluble catalyst to form oxygenate compounds, and then catalytically producing a liquid fuel from the reformed solution. In U.S. Pat. No. 8,940,060, a method is described for forming a pyrolysis oil wherein the feed biomass is washed with a portion of the pyrolysis condensate to produce a washed biomass having a reduced level of metals, and thermally pyrolyzing the washed biomass. Catalytic reaction is not discussed.
Experimental results have been presented (see V. Paasikallio, C. Lindfors, E. Kuoppala, Y. Solantausta, A. Oasmaa, “Experiences from an extended catalytic fast pyrolysis production run”, Green Chem., 2014, 16, 3549-3559) in which the amount of ‘Alkalis’ deposition as a function of time on stream in a CFP process showed a linear increase with time. ‘Alkalis’ are defined to include K, Ca, Mg, and P. After a four day test of pine sawdust catalytic fast pyrolysis with H-ZSM-5 catalyst, the catalyst had accumulated 1.1 weight % of the ‘alkali metals’ including K, Ca, Mg, and P. The acidity of the catalyst decreased and the O/C ratio of the produced bio-oil increased, which were interpreted to indicate a reduction of catalytic activity. No attempts to remove alkali metals from the feed or from the process were discussed.
Oudenhoven et al in “Demineralization Of Wood Using Wood-Derived Acid: Towards a Selective Pyrolysis Process for Fuel and Chemicals Production” J Anal Appl Pyrolysis 103 (2013) 112-118, describe the use of a raw pyrolysis water product phase to wash biomass prior to a thermal pyrolysis. Increased yields of bio-oil rich in oxygenated products, i.e. levoglucosan, are reported for the washed wood experiments. Catalytic pyrolysis or the production of aromatics was not discussed. By contrast, Kasparbauer in his PhD thesis entitled “The effects of biomass pretreatments on the products of fast pyrolysis” (2009), Graduate Theses and Dissertations, Paper 10064 at Iowa State University, concludes on page 127 that: “The water wash pretreatment showed no significant difference when compared to unwashed biomass in terms of product yields.”
It has been often reported that improved yields of useful products are obtained when AAEMs are introduced into, or not removed from, biomass. U.S. Pat. No. 5,865,898 describes a process for “pretreating a lignocellulose-containing biomass comprising the steps of adding calcium oxide or hydroxide and water and an oxidizing agent to the biomass” to obtain better yields of sugars, ketones, fatty acids, and alcohols.
Wang et al have reported that AAEMs reduce the yields of aromatics and olefins in ex situ catalyzed pyrolysis reactions in “The deleterious effect of inorganic salts on hydrocarbon yields from catalytic pyrolysis of lignocellulosic biomass and its mitigation”, Applied Energy 148 (2015) 115-120. Their studies used separate pyrolysis and catalytic upgrading reactors to show that pretreatment of the AAEM-infused cellulose can improve aromatics and olefins yields. No attempts were made to react biomass in the presence of a catalyst in a single reactor.
Among other methods of pretreating biomass, wet milling of corn is routinely used in the industry to separate the various components. Typically the hemicellulose and cellulose are hydrolyzed for further upgrading to ethanol or other products. Wet milling is not used for minerals removal. As it is applied in extracting sugars from corn, wet-milling is a process in which feed material is steeped in water, with or without sulfur dioxide, to soften the seed kernel in order to help separate the kernel's various components. The hydrolysis of the hemicellulose and cellulose is detrimental for a feed that will be upgraded by the CFP process of the present invention.
U.S. Pat. No. 7,503,981 teaches the removal of minerals from biomass as part of a biomass saccharification process that produces dimeric and monomeric saccharides (sugars) from cellulose and hemicellulose using sulfuric acid.
Pretreatment of biomass has been developed broadly for the production of monomeric sugars as precursors in fermentation processes to produce ethanol. These pretreatment processes are optimized for the hydrolytic deconstruction of cellulose and hemicellulose, separation of lignin, and the removal of contaminant materials to provide a sugar rich solution for fermentation. For a catalytic fast pyrolysis process in which all of the cellulose, hemicellulose, and lignin contribute to the yield of valuable materials such as BTX, the processes adapted for ethanol are not applicable since in the production and separation of the sugars a very significant amount of organic material is lost in the lignin and other minor components. The yields of BTX obtainable from these deconstructed feeds in a CFP process are fundamentally limited by the loss of carbon.
Conversion of wood or other cellulosic feedstocks into paper has been commercial for more than a hundred years. The Kraft process is the dominant process used to convert wood into wood pulp, which consists of almost pure cellulose fibers. Wood pretreatment processes have been developed to improve the quality of the wood pulp obtained in the subsequent Kraft process. For example, Lundquist et al in “Removal of Nonprocess Elements From Hardwood Chips Prior to Kraft Cooking,” presented at the 59th Appita Conference, 16-19 May 2005, in Auckland, New Zealand, reported that a 24-hour acid leaching of birch or eucalyptus chips in sulfuric acid solution of pH 2.5 at room temperature (22° C.) resulted in thorough removal of K ions and partial removal of Ca ions. However, the extremely long leaching times required make the process unacceptable for large scale, continuous or semi-continuous manufacture of chemicals such as BTX.
In light of current commercial practices and the disclosures of art, a simple, economical, rapid process for enhancing production of aromatic compounds, such as, for example, benzene, toluene and xylenes, from a catalytic pyrolysis process utilizing biomass containing impurities such as alkali and alkaline earth metal components, sulfur compounds and/or nitrogen compounds is needed. The present invention provides such a process.